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Hi,
Just wondering if anybody tried to reverse engineer vht power amps like 2150, 2902, etc. There's just something that's been bugging me.
I know they all tend to use buffered cathodyne PIs. I hear a lot of claims that say 2150 and 2100 use the second tube (12at7) as AC coupled CFs, but numbers just don't add up.
The manual claims sensitivity up to -10dbV, which is 0.316V rms or 0.447V amplitude.
Now, KT88 would probably need what, around 75V bias, give or take? That means the previous stages need to provide gain close to 168, and that is in a closed loop (i.e. after gain reduction due to nfb).
So, the first 12ax7 is likely a gain stage followed by cathodyne. Considering the most usual place for nfb is the cathode of the gain stage, it can't have its cathode resistor bypassed. This means the stage is unlikely to have gain more than 50 (at a push). Then cathodyne gain is just under unity, and so is CF's. Clearly there's not enough drive to give all the swing required, and this is even before one looks at nfb.
Now, manuals for 2902 and 2502 mention that their second tubes (12au7) are "voltage amp/driver", which would suggest they are common cathode gain stages, rather than cathode followers. So, does this mean that in 2150 the second tube is actually a pair of post-PI gain stages, rather than CFs, as people lead to believe?
Or am I missing something?
Thanks,
Konstantin
Just wondering if anybody tried to reverse engineer vht power amps like 2150, 2902, etc. There's just something that's been bugging me.
I know they all tend to use buffered cathodyne PIs. I hear a lot of claims that say 2150 and 2100 use the second tube (12at7) as AC coupled CFs, but numbers just don't add up.
The manual claims sensitivity up to -10dbV, which is 0.316V rms or 0.447V amplitude.
Now, KT88 would probably need what, around 75V bias, give or take? That means the previous stages need to provide gain close to 168, and that is in a closed loop (i.e. after gain reduction due to nfb).
So, the first 12ax7 is likely a gain stage followed by cathodyne. Considering the most usual place for nfb is the cathode of the gain stage, it can't have its cathode resistor bypassed. This means the stage is unlikely to have gain more than 50 (at a push). Then cathodyne gain is just under unity, and so is CF's. Clearly there's not enough drive to give all the swing required, and this is even before one looks at nfb.
Now, manuals for 2902 and 2502 mention that their second tubes (12au7) are "voltage amp/driver", which would suggest they are common cathode gain stages, rather than cathode followers. So, does this mean that in 2150 the second tube is actually a pair of post-PI gain stages, rather than CFs, as people lead to believe?
Or am I missing something?
Thanks,
Konstantin
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